Laser assisted etching of semiconductor substrates and some of the inherent problems therewith have been previously described by Bjorkholm, J. E. and Ballman, A. A., in an article entitled "Localized Wet-Chemical Etching of InP Induced by Laser Heating" Applied Physics Letters Vol. 43, No. 6 (1983), pp. 574-76 ("the Bjorkholm et al. article"). The Bjorkholm et al. article describes the etching of grooves in indium phosphide samples mounted vertically in a quartz curette containing an etching solution of aqueous H.sub.3 PO.sub.4.
The Bjorkholm et al. article describes the effects of laser power and scan rates on the laser etching of continuous cross sections of grooves in an InP sample. The article identifies a potential problem attributable to gas bubbles generated when the grooves are etched. The gas bubbles are potentially troublesome because they tend to cling to the etched grooves and produce a barrier between the substrate sample and the etchant solution which causes laser light which is subsequently applied thereto to be scattered and defocused. The area in which the laser beam is applied on the sample is sometimes referred to as "the laser reaction zone". The Bjorkholm article describes the conditions under which bubble formation was found to be a problem. For sufficiently high scan rates, which allow the laser beam to "run away" from the bubble, etching was not impeded. However, in the case of stationary etching as required for via hole formation, or scan rates for making deeper grooves, the bubble formation can stop the etching process.
The gaseous bubble barrier in the laser reaction zone causes scattering and defocusing of the incident laser beam, and significantly reduces or can even stop the etching process in the area beneath the gas bubble. This problem is especially troublesome in cases where a slow scan rate or no scan rate, e.g., in via-hole etching, of the incident light source is used, and also in cases where higher power lasers are used which promote thermal heating and the generation of gas bubbles. Slow scan rates are used in cases where, unlike the shallow grooves described in the Bjorkholm et al. article, a groove having a high aspect ratio is desired. In such cases the laser cannot simply run away from the forming gas bubbles and the process is interrupted by the forming gas bubbles.
In addition to slowing the etching process, the scattered light produced by the gas bubbles makes it difficult to produce small closely spaced via holes because the scattered light causes etching of a greater surface area of the substrate than is desired. In certain applications, e.g., fabrication of MMICs, small closely spaced via holes are desirable in order to achieve low-inductance grounding. Low-inductance grounding may be achieved by using the via hole technique which allows the placement of grounds through the substrate. The necessary holes are back etched through the substrate until the front metallization pattern is reached. These holes are subsequently metallized simultaneously with the ground plane to provide a continuity between ground plane and the desired frontside pads of the metallization pattern. These frontside pads are usually small and very closely spaced apart. This makes it essential for the via holes connecting the front pads to the back ground plane to be small throughout their length.